Light is a powerful tool in many of today's most widely used life science instruments, including microscopes, endoscopes, analytical instruments, diagnostic instruments, medical devices and miniaturized analyzers. Reliable high intensity, low cost light engines are essential to the design and proliferation of these life science instruments.
Lighting for life sciences is a broad and general category. The specifications for the power and spectral content of the light are varied and so too are the equally important optical delivery requirements. Spectral and spatial lighting requirements for sensing on the head of an optical probe or within a single cell in a flowing stream differ in output power by orders of magnitude from the requirements of a multi-analyte detection scheme on an analysis chip or within the wells of a micro-titer plate. The number of colors, spectral purity, spectral and power stability, durability and switching requirements are each unique. Illuminating hundreds of thousands of spots for quantitative fluorescence within a micro-array may be best served by projection optics while microscopes set demanding specifications for light delivery to overfill the back aperture of the microscope objective within optical trains specific to each scope body and objective design.
Arc lamps are noted to be flexible sources in that they provide white light. The output is managed, with numerous optical elements, to select for the wavelengths of interest and, for typical fluorescence based instruments, to discriminate against the emission bands. However arc lamps are notorious for instability, lack of durability, large power demands, large size, and significant heat management requirements, which make them less than ideal for life science instruments and particularly portable instruments.
Lasers can provide high power coherent light in particular colors dependent upon their design. Lasers require a trained user and significant safety precautions. While solid state red outputs are cost effective, the shorter wavelength outputs are typically costly, require significant maintenance and ancillary components. Color balance and drift for multi-line outputs is a serious complication to quantitative analyses based on lasers. Moreover, the bulk of fluorescence applications do not need coherent light, are complicated by speckle patterns and do not require such narrow band outputs. Overcoming each of these traits requires light management and adds cost to the implementation of lasers for use in life science instruments.
LEDs (light-emitting diodes) have matured significantly within the last decades. LEDs are now available in a relatively wide range of wavelengths. Their output is broad, but, output in the visible spectrum is profoundly reduced in the green wavelengths, 500-600 nm (the so called “green gap”). LEDs presents trade-offs with respect to emission wavelength dependent intensity, broad emission spectrum (spectral half width on the order of 30 nm or more), poor spectral stability, and the wide angular range of emission. In addition, the process used to manufacture LED's cannot tightly control their spectral stability; anyone wishing to use LED's in applications requiring a good spectral stability typically works directly with a supplier to essentially hand-pick the LED's for the particular application. Moreover the spectral output of an LED varies with temperature. Also, LED's emit light over a wide angular range (50% of light intensity emitted at 70°). While optics can narrow the emission band and focus the light output, the resulting loss in power and increase in thermal output further complicates the use of LEDs in light engines.
Most importantly, the fundamental light source technologies (e.g. lasers and LEDs) cannot be readily improved for bioanalytical applications. The light engine market simply does not justify the large investment necessary to overcome fundamental performance limitations in the lasers and LEDs themselves. Moreover the numerous manufacturers of lamps and lasers provide only a source, not an integrated light engine. Companies such as ILC Technology, Lumileds, Spectra-Physics, Sylvania and CoolLED, Ltd. produce light engines which require some sort of mechanics and or electro-optics such as acousto-optic tunable filters (AOTFs), excitation filters (with a wheel or cube holder), shutters and controllers. As a result, the performance and price of life science instruments instrument is constrained by the available light source technologies and light engines which utilize them. Accordingly there is a need for solid state light engines which overcome the limitations of the present technology.